Method of and apparatus for giving heat treatment to magnetic metals.



- R. B. FEHR.

METHOD OF AND APPARATUS FOR GIVING HEAT TREATMENT T0 MAGNET") METALS.

' APPLICATION FILED SEPT. 28, 1914. T

1,1 88,430. Patented June 274916.

ROY BENNER FREE, OF STATE COLLEGE, PENNSYLVANIA.

METHOD OF AND AIPPA apparatus for heat treating vmetals such as iron and its alloys to improve its physical and chemical structure for commercial uses. Carbon steels such as hold in solid solution iron carbids in the form of austenite, martensite, troostite, osmandite, or pearlite depend for its physical and chemical properties upon the heat treatment given it. When it is cooled from a molten condition its cooling curve has a sharp jog -known as the recalescent point where heat is given up by molecular changes in the ferrite and carbids as constituents. Conversely, when the steel is heated up reactions again occur in a reverse sense andheat is absorbed; this is known as the decalescent point. The usual treatment consists in heating it through its critical range represented by the two points mentioned and noting by the indications of a pyrometer the temperature, to ascertain when these points are attained. The decalescent and recalescent points do not coincide on account of hysteresis and,other causes within from 20 to 50 degrees centigrade, the decalescent point always being the higher, and as the pyrometer occasions an instrumental error by reason of thermal lag and close calibrating scale the exact moment when the steel reaches the critical tempera- 40 ture is exceedingly difficult to determine with certainty. This is accentuated because different instruments are in practice used in the laboratory and in the shop, and their calibration is not uniformly standardized. The higher the temperature above the decalescent point from which the steel cools the larger the size of its grain, and the grain, texture, tensile strength, ductility and other hysical and chemical characteristics depen on the accurate location of the critical point. By reason of the difliculty in practice of accurately locating the jogs in. the heat curve a liberal allowance is made in excess of the required temperature, to approach as nearly as 5 possible to the known required temperature.

Thus the present practice is 'in reality only Tus FOB GIVING- HEAT TREATMENT To MAGNETIC METALS.

an afiProx-imation toward a perfect result, and there is not the certainty desired as to the crystallization or as to the character of the carbids, or as to the allotropic form of the ferrite containing it! By m invention the results are rendered very de 'nite, as the decalescent point can be quickly and very accurately determined. I determine the proper point for heat treatment by passing a magnetic flux through the iron or steel while heating and locate the jog in the heating curve instantly, when it occurs, on an electrical indicating instrument inductively related to the magnetic lines of force threading the iron or steel. The magnetic criticalpoint is at about 760 C. in steels of 0.30 carbon and for steels having more than 0.3% carbon this critical point decreases slightly but in any case this point.

marks the boundary of the alpha form of iron. The point can be regarded as the proper one for refining or quenching all steels that are given heat treatment. his point is coincident with the decalescent" point and the molecular change in the iron which marks the jog of the heating curve is undoubtedly the origin of the change in the magnetic curve. The point is the one where pearlite goes into solid solution.

Recent accurate tests of certain kinds of steel show conclusively that difi'erences of a few degrees (10 to 30) may alter the properties of steel greatly. It will be apparent, therefore, that a direct method of accurately determining the critical point when the" metal is finally treated is of prime impor- -tance.

specification of Letters Patent. Patentgdfiune 2?, 1916. Application filed September 28, 1914. I Serial No. 888,899.

the magnetizing current produces a large deflection in a milli-voltmeter inductively related to the magnetic circuit. The voltmeter deflections are of large magnitude and there is no chance of error in observation when the decalescent point is thus electromagnetically determined. When thepoint is thus accurately found I quench, temper or anneal the metal according to its required commercial use and secure a product of the best and most uniform nature to meet the desired specifications. I thereby can accurately control the grain, by removing the metal from the furnace at the-exact instant necessary.

My invention therefore consists in treating magnetic metals to accurately locate their critical points of heat treatment by creating at will a magnetic flux inthe metal while under heating and definitely -indicating the point by magnetic agencies. It comprises also apparatus for carrying out the method embodying means for operating an indicating instrument at will during the heat treatment in accordance with the changes in magnetic flux at different temperatures to locate the critical point for heat treatment. Other features of novelty will be hereinafter 'more fully described and will be definitely indicated in the appended claims.

' In the accompanying drawing illustrating the invention, the figure shows diagram-. matically an apparatus by which the invention may be practised. v

P is a piece of steel or any magnetic metal to be treated. For urposes of illustration this is shown as an ingot; it may, however,.

have .any form desired, as that of a tool, an axle, a plate, avrod, or infact any form where high grade steel is required. The article is surrounded by a heat chamber, as a furnace, or may be electrically heated in any known way, Electrical control is referred by reason of its convenient adustment, but other modes of heating could be employed. 4

Asshown the furnace 4 is heated electrically by a heat-refractory high resistance ribbon, such as nichrome which. may be embedded in the furnace walls in the usual way or may be wound ona refractory insulating or insulated tube, as indicated. The

= ribbon heater is wound non-inductively "and the consecutive turns distributed to roduce uniform temperature conditions in t e tube as by graduating the distance between conat 5. The magnetizing current may be fursecutive turns toward the ends. Adjust ment of the heatlmay be controlled in usual ways by a regulating rheostat, as indicated nished by a copper wire surrounding the furnace, but the plan shown is more convenient being a coil of wire wound around an iron yoke 6 which terminates at the sides of the furnace, the metalto be treated being'placed with firm contact between the jaws of the yoke, any space between its ends being filled with soft ironflbridge pieces 7, 7. An adjustable resistance 13 is inserted in the magnetic circuit by which the strength of the magnetic field may be regulated. An ammeter is included in the magnetizing circuit, as indicated at 8. The current may ,be derived from any suitable source as indicated by the signs plus and minus at the supply mains. In the'circuit is included. a reversing switch 9. Thus in the magnetic circuit.

while the article is being heated while carrying ,magnetic flux the reversing switch may bemanipulated. to 'break and reverse the magnetizing current, creating a maximum range of variation of the magnetic flux. I have shown a milli-voltmeter 10 connected with a copper coil 11 wound around the magnetic circuit. The circuit includes a control-switch 12. In operating, to find the critical point of the heat curve, the exciting current is adjusted by the rheostat 13 until the ammeter reading is of a proper value to produce, a flux of suitable density The switch 12 is then closed and the reversing switch 9 is opened; the momentary large deflection of the milli-voltmeter is then read. This read ing is proportional to the change in magnetic flux from, say, plus 14,000 units with the magnetizing current to aresidual flux of say plus 7,000 units with no magnetizing corresponding to a change in flux from plus 7,000 to minus 14,000 units. These operations are repeated at regular intervals during the heating process, and thus there may be obtained two sets of observations, one set for breaking the circuit, and the other for making the circuit. One set would be suflicient, but the other set gives a reliable check on the work. When the readings in either set suddenly begin to increase or decrease (according to whether'the magnetic induction is'above or below the knee of the saturation curve of steel) I have a positive indication that the metal in the hottest part of the furnace is becoming non-magnetic and ready for heat treatment.

' I It should be noted that switch 12 in the secondary circuit is only for the purpose of preventing injury to the pointer of the milli-voltmeter, for when switch 9 is opened from its reversed. position, the milli-voltmeter pointer would be deflected in the oppositedirection, z. -e. off itsscale. It is therefore advisable to use a milli-voltmeter with its zero at the middle of the scale. In

' this case switch 12 can be omitted.

The repeated observations of the magnetic condition of the steel during the heating are of great importance in order to determine accurately the exact moment when the metal should be removed from the furnace for annealing, tempering, or hardening. It requires a sensible interval of time (several minutes) for all the metal to pass the critical temperatures, because the phenomenon of decalescence itself requires time, and especially because all parts of the furnace are not at the same temperature. Uniformity of heating throughout the mass of metal is of the greatest importance in heattreatment, for unless the steel has-a uniform temperature at the time of annealing or quenching, the grain-size, constitution and physical properties will vary in the same piece vThis is particularly true of the larger pieces of steel that are heattreated. If the galvanometer deflections re'-' ferred to above show only .a gradual change it is safe to conclude that the furnace .does not heat uniformly and in such case the furnace should be rebuiltijwith a view toward equalizing its temperature. My point to point method of determining the critical temperature for heat-treating has a decided advantageover previouslmethods in that it indicates whether or not the steel has been heated uniformly past the critical temperature.- a

Having thus described my invention what I- claim as new and desire to secure by Letters-Patent is:

ature, causing magnetic flux topass through 4 it from a source outside the region of heat, taking repeatedly and instantaneously indi cations of the flux changes when the steel is passing the decalescent point, and then quenching, annealing or tempering. I V

3. Apparatus for treating steel comprising means for heating it through the decalescent point, a magnetic circuit extending away from the source of heat, a coil induc- ,tively related to the magnetic circuit remote from the source of heat, and a dead beat indicator controlled by the coil for taking repeated instantaneous indications of the magnetic condition whilethe steel is passing through the decalescent point.

- 4. Apparatus for heat treating steel comprising means for heating. it through the decalescent point, a magnetizing coil around i the magnetic circuit external. to the source of heat for creating magnetic flux in the magnetic circuit including the-steel to be treated, means for varying the magnetizing current, and means for indicating-repeatedly and instantaneously the loss of magnetic properties of the steel.

5. Apparatus for heat'treating a magnetic metal comprising means for heating it through the decalescent 'point, means for supplying magnetic flux imposed outside the region of heat, "means for interrupting the flux, and means for indicating. the difference between the total magnetic induction in the metal and the residual magnetism.

Signed at State College, in the countyof Center and State of Pennsylvania, this 26th day ofaseptember, A. D. 1914. a

ROY BENNEB FEHR;

Witnesses:

V. R, Jones, Joni: -L. NEcnNEn. 

